Buffs Needed:
Mass Deathward
Bard Songs
Prayer
Haste
Rage
Sonic Resist (Orthon's)
Fire Resist (Pit Fiend, Fire Elemental)
Fire Protection
Poison Resist
True Seeing
Freedom Of Movement (Earth Elemental)
Blur
Displacement
Greater Heroism
Magic Circle Against Evil
Shield of Faith Mass
Mass Aid
Mass Spell Resistance (Gnoll)

Offensive Spells:
Cone of Cold - On Pit Fiend
Acid Fog - Minimal Damage, Helps Slow Him

Weapons To Use:
Various - The 4 Mobs from Part 2 will be your first enemies here.
Transmuting of Pure Good
Transmuting Silver
Silver of Pure Good
Transmuting Silver of Pure Good

Scrolls/Pots:
Fire Protection
Major Mneumonics
Heal Scrolls

Everyone should really be carrying Fire Pro Pots.  They help your healers out by mitigating damage you may receive from a meteor to the face.

Tips:
The Pit Fiend on Normal has DR 15 against Good, and DR 15 against Silver, with Light Fortification.  You must bypass both DR's or you will get Yellow Damage numbers.

There are pools in here that you can use to replenish your spell points.  They will dry up as the fight wages on.

Assignments:

Caster for Haste
Healing Order for Mass Heals
1 Kiter for initial Zone In.

Healing Strategy:
Healers take turns mass healing the party.  One tank is called as the healing tank, all heals will be on them.  Take Aeritrikos by the alter, so the healer can remain in the pool moving in and out while healing, to extend his mana usage as far as it can go.  Once the first healer is out of mana, the 2nd kicks in, while the first moves to a new pool to get his mana back.  This continues until Aeritrikos is down.

General Strategy:

An order should be made on the death list, who dies 1st, 2nd, 3rd, 4th.  All the mobs from Part II are in here.  You want to take certain mobs down first.  Gnoll should be a primary, Fire, Devil, etc..  Leave 1 mob up, like the Trog, Kobold, Ogre, that you can block in.

As you zone in, you will be dead.  All but the Kiter move to the NW Corner.  As the DM text comes up for "Destroy Them My Lieutenants", the whole party with the exception of the Kiter runs to the NW pool.  The Kiter then grabs all aggro and runs them around on the East side of the map.  Get buffed up, and pull the mobs off the Kiter 1 at a time, in the order you determined.  Once the last mob is standing, you can block him into the corner by the alter, on the west side, up against the purple shield.  Have the kiter jump out after he is blocked in, and let the caster/clerics get their mana back.  Once everyone is filled up, the last mob can be taken down.  Once down, the Pit Fiend will appear.  Move to the NW corner of the alter platform.  Let the Fiend come to you.  Once he is close to the corner, surround him, do not let him out!!  Healing rotation should begin.  At 1/4 health, portals will appear, and whirling blades will start to come out of them and wander about.  They are attracted to movement, and will follow you around if you try to run away.  Just be careful of them.  At 50% health the pools will start drying up.  As the fight wears on, if it takes to long, the pools will continue to dry up.  Healing rotation should be kept up until the fiend is down.

That's pretty much it.

Again this is all in the opinion of the author, so please, if you have other techniques that work, by all means use them!!